This invention relates to the manufacture of cathode ray tubes for television receivers. The invention comprises an improvement in an apparatus for testing the hermetic qualities of the seal interposed between the front panel and the funnel of a cathode ray tube. The apparatus is that described and claimed in U.S. Pat. No. 3,943,438, assigned to the assignee of the present invention. The substance of the disclosure of U.S. Pat. No. 3,943,438 is fully incorporated herein by reference.
The apparatus disclosed in the referent patent provides for first stretching then relaxing an elastomeric band over the hermetic, glass-like frit seal of an in-process cathode ray tube. The inner conductive coating of the cathode ray tube is charged to a high test voltage, while the band is established at ground potential. A discontinuity in the hermetic seal causes a high voltage breakdown at the point of discontinuity when the test voltage is applied. The breakdown activates appropriate circuits which shut off the high voltage and provide an indication of the breakdown condition.
The elastomeric band is made conductive by impregnating it with graphite. A fairly high concentration of graphite is required to ensure minimum electrical resistance so that the band provides an adequate ground at all points of contact with the frit seal, and does so without introducing a high resistance into the electrical test circuit. However, the amount of graphite required for adequate electrical conductivity also renders the band brittle and subject to rupture. If the band is made more elastic by the addition of less graphite, the resistance of the band will be such as to render the testing ineffectual at points distance from the place of introduction of the ground potential and any possible discontinuity.